1. Field of the Invention
The present invention relates in general to pulverizers for pulverizing coal, and in particular to a new and useful annular passage design for such pulverizers which has an improved low pressure drop characteristic.
2. Description of the Related Art
One type of known coal pulverizing mill (pulverizer) is a slow speed, roll-and-race-type pulverizer that uses three large-diameter grinding rolls to crush the coal. Primary air enters the pulverizer through a radial inlet duct, moves into a low-velocity air plenum, and is then accelerated and oriented by a series of stationary passages in a ring that surrounds the grinding zone. At the outlet of the passage, the pulverized coal particles are entrained by the high-speed airflow. The velocity of the air is then reduced in the main pulverizer housing causing the larger particles to be returned directly to the grinding zone for further crushing, while the smaller particles are carried up through the classifier for final sizing.
A large portion of the primary air pressure drop is due to losses across this annular passage. In some known pulverizers, the primary air pressure drop can be about 40% higher than other mills containing different, annular passage designs. Due to this higher pressure drop, more fan power is required to operate the mill. This results in a large power penalty due only to the annular passage design.
A known design illustrated in FIGS. 1 and 2, is a modified version of the earlier stationary annular passage design of the early 1980's. This design consists of forty-two passage ports (10) made up of fourteen separate castings mounted to the top and bottom of a grinding table (16). The annular passage is divided into the individual passage ports (10) through the use of flow vanes (12). The vanes extend from the annular passage inlet (10a) to the annular passage outlet (10b) and are included at an angle (14) of 30.degree. from the horizontal and an angle (18) of 15.degree. from the vertical toward the grinding zone (20). The outer annular passage wall (22) is stationary while the remainder of the annular passage including its inner wall (24) and the vanes (12) is rotated with the grinding table (16). The air flow is initially oriented by a tear-drop shape (12a) at the leading edge of the vane (12) and is accelerated to promote a uniform velocity profile over an airfoil shape (12b) on a portion of the upper surface. Table (16) rotates within a housing (6), about a vertical axis. Wall (22) is supported in the housing and the housing encloses the grinding zone (20). The function of the vanes is to accelerate and orient the flow through the annular passage as described in U.S. Pat. No. 4,264,041.
Other pertinent existing prior art relating to pulverizer passage designs are U.S. Pat. No. 2,275,595 (Schwartz '595); U.S. Pat. No. 2,378,681 (Bailey, et al '681); U.S. Pat. No. 2,473,514 (Ebersole '514); and U.S. Pat. No. 2,545,254 (Bice '254), all of which are assigned to The Babcock & Wilcox Company. Schwartz '595, discloses curved annular passages fanning a annular passage discharging scavenging air in the direction of the grinding elements. Bailey, et al '681, discloses a design for constant air velocity through the annular passage. Ebersole '541, discloses an adjustable annular passage, and Bice '254, discloses an eccentric passage design for air distribution.